The present invention relates to a method for detecting the position of a predetermined member using a position detecting mark formed on this member.
A position detecting method of the vibration type is used for a photoelectric microscope and has high detection sensitivity; therefore, it is nowadays used as a position detecting method for various kinds of automatic positioning apparatus such as a semiconductor manufacturing apparatus and the like. Such a method is disclosed in, for example, the literature (Nikon Tech. J., No. 2,24, 1973, "About Photo-electric Microscope") (The first volume of Servo Technology Manual &lt;New Technology Development Center&gt; III-72).
The principle of the position detecting method of the vibration type will now be simply described. In this method, a light beam from a beam radiating source is radiated onto an object under examination on which a mark is formed and the reflected light is detected by a detector through a vibrating slit. In this case, since the slit is vibrating, the waveforms of the detection signals differ depending on the relative position between the slit and the mark. A change in signal waveform to the mark position X is as shown in FIG. 12. As will be understood from this diagram, when the mark position coincides with the vibrational central position of the slit, namely, at point (e), the detection signal becomes the signal having a frequency, which is twice the oscillation frequency (fundamental frequency) of the slit, and the fundamental frequency component becomes zero. Therefore, only the fundamental frequency component is obtained from the detection signal by way of synchronous detection, thereby removing the component having a frequency which is twice the fundamental frequency.
The state of change of the fundamental frequency component to the mark position in this case is shown as an output characteristic curve. This output characteristic curve is suitable for use in the detection of the position by means of a zero-meter, and the driving of a servo system because the output becomes zero at the point where the mark position coincides with the vibrational center of the slit and the signs before and behind that point are opposite.
On the other hand, the shape of the above output characteristics curve varies depending on the position detecting conditions such as the mark width, slit width, contrast, scanning (oscillating) amplitude, brightness of illumination, etc., so that the relation between the detected value and the mark position is not made constant. Therefore, in order to always obtain correct position information, it is necessary to perform the calibration every time. If calibration is not performed every time, position setting will become inaccurate and it will take a very long time to detect the position. For instance, the case will be considered where this position detecting method is applied to the detection of position prior to exposure by a light exposing apparatus for use in a process for manufacturing semiconductors. Since the exposure is repeatedly performed before and after a number of various kinds of processing steps in the semiconductor manufacturing process, the contrast of the mark for detection of the position differs every time due to the difference in steps and the difference in resist conditions and the like, so that the peak value and gradient of the output characteristic curve of the photoelectric microscope of the vibration type will be different each time. Due to this, a method has also been considered whereby the information regarding the contrast of a mark depending upon the difference in steps is preliminarily stored in a computer and the output is corrected on the basis of this information. However, in this method, it is impossible to correct with high accuracy since the contrast information of a mark is based on the presumption. Further, since this output also becomes unstable due to variation in illuminance, when sufficient stability of illuminance cannot be obtained, the peak value and gradient of the output characteristic curve also change with the lapse of time. In addition, recently, it is possible to perform the alignment using the principle of this position detection for an electron beam transfer printing apparatus which is considered to be promising for the transfer printing of fine patterns in the submicron order. However, a similar problem is also caused in this case. Moreover, in this case, although an electron beam is radiated onto a mark in place of radiating the light beam thereon, there is the phenomenon of a photoelectric surface, which serves as a source for generating this electron beam, and which deteriorates with the elapse of time, causing the quantity of radiation of the electron beam to change as the time passes. Thus, the time change also appears in the output signal. Since such time changes are difficult to forecast, it is actually difficult to correct the output on the basis of the above presumption with high accuracy.
For these reasons, it is necessary to set the gradient of the output characteristic curve to always be constant by means of the AGC (auto gain control) of the signal. It is simple for the AGC to control the gain so that the peak voltage of the output characteristic curve becomes a constant value by always making a mark for detection of the position approaching the beam from a fixed direction. However, in this method, it is necessary to perform the detection while the mark is being scanned for a certain time interval, so that it takes a long time to detect the position, reducing the throughput of the apparatus. Moreover, this method cannot cope with the time change of the signal during the time period when the peak value of the output curve is detected and it is difficult to perform the precise positioning within a short time.
On the other hand, as a method by which AGC is performed in real time, a method has been considered whereby the effective value or peak value of the signal is detected and the gain is controlled so that the value is always constant. However, the detection signal obtained by the vibration type position detecting system has a complicated signal waveform because it includes the high frequency components as well as the fundamental frequency component, and their output components largely differ in their dependence upon the mark positions, and the like. Consequently, in the conventional method, it is difficult to perform AGC with a high degree of accuracy without using position information.